1. Field of the Invention
The present invention relates to a rotary type electric shaver in which an outer cutter has a shaving surface on the upper surface of a ring-shaped thin layer portion thereof, and an inner cutter rotates while making sliding contact from below with the lower surface of the thin layer portion, thus cutting, with the inner cutter, whiskers or hair that enter into a hair introduction hole formed in the thin layer portion.
2. Description of the Related Art
In a type of electric shaver as described above, when the hair introduction hole (hereinafter “slit”) that is a slit or the like formed in the thin layer portion of the outer cutter is firmly pressed against the skin during shaving, the skin enters from the slit and is cut too deeply by the rotating inner cutter and causes a burning feeling in the skin after the shaving. In order to prevent this sort of problem from occurring, it is obvious that the slit width can be made narrow or the outer cutter can be made thick. Here, making the outer cutter thick means increase in dimensions in the thickness direction of the ribs are located on both sides of the slit (the dimension in the direction parallel to the center axis line).
Japanese Patent Application Laid-Open (Kokai) No. H7-185148 teaches a solution to this problem, teaching that the upper surface (shaving surface) and lower surface of a ring-shaped thin layer portion formed in an outer cutter are shaped into convex upward in a vertical cross section that is a cross section in a radial plane that includes the center axis line, and the radial and vertical cross-sectional shape of the outer cutter is an arc shape that is substantially of constant thickness. This prior art improves the shaving feeling by appropriately setting the curvature of the thin layer portion of the outer cutter. More specifically, it sets the dimensions of a straight line connecting two points on the upper surface of the outer cutter corresponding to the inner and outer diameter of the inner cutter and the highest position on the upper surface of the outer cutter (apex) in a predetermined range.
When the thin layer portion of the outer cutter is curved convexly upward as disclosed in Japanese Patent Application Laid-Open (Kokai) No. H7-185148, the rigidity of the outer cutter increases, and it is possible to increase the pressing force of the outer cutter against the skin. Furthermore, the region near the center (or apex area) of the thin layer portion upper surface of the outer cutter (more specifically, the shaving surface) projects upward; as a result, when the shaver touches and presses the skin, it touches there firmly, and the pressing force becomes weak at positions distant from the center. Therefore, the problem is that if the slit width in the outer cutter is made constant, the skin near the center of the shaving surface enters the slit deeply, causing too deep shaving, and a burning feeling in the skin after shaving.
In order to prevent this sort of problem, if the slit width is constant, then the thin layer portion of the outer cutter can be made sufficiently thick as described above. However, if the thin layer portion is thick, at portions where the pressing force of the outer cutter against the skin becomes weak (or at portions distant from the apex of the thin layer portion), and it is not possible to shave deeply enough; and another problem occurs that the shaving feeling is unpleasant.